1. Field of the Invention
The present invention relates to a heat transfer tube having an improved heat transfer performance for use with an absorber in an absorption refrigerating machine, absorption cooling and heating machine, etc.
2. Description of the Related Art
An absorption refrigerating machine employs a solution of lithium bromide (LiBr) which is a part of salt as an absorbing liquid and water as a refrigerant. Water vapor in the refrigerating machine is absorbed by a strong moisture absorbing power of the lithium bromide to reduce pressure in the refrigerating machine, and the water as a refrigerant is vaporized at a low temperature to thereby make a cold water to be used for air conditioning or the like.
In an absorber of the absorption refrigerating machine, a plurality of heat transfer tubes are horizontally arranged. Water is fed to flow in the heat transfer tubes, and the absorbing liquid is dripped onto the outer surfaces of the heat transfer tubes, thereby performing heat exchange between the water and the absorbing liquid. A smooth tube is conventionally used as each heat transfer tube. The absorbing liquid dripped flows down on the outer surface of the smooth tube at a low velocity, that is, in the state of laminar flow. Accordingly, a surface active agent is added to the absorbing liquid to generate Marangoni convection upon absorption of water vapor, thereby promoting the turbulence of the absorbing liquid to increase the absorption efficiency.
FIG. 5 is a fragmentary sectional view of a heat transfer tube 1 in the absorber, showing the condition of generation of the Marangoni convection. An absorbing liquid 2 containing a surface active agent is deposited on the outer surface of the heat transfer tube 1. The absorbing liquid 2 is raised in the direction perpendicular to the axial direction of the tube 1 in such a manner that a plurality of ridges 3 extending in the circumferential direction of the tube 1 are formed in the axial direction of the tube 1. These ridges 3 are waved in the axial direction of the tube 1 and simultaneously flows down as shown in FIG. 6. The Marangoni convection is a convection generated by making surface tension of the liquid nonuniform. The generation of the Marangoni convection causes the formation of the ridges 3 of the absorbing liquid 2 in the flow-down direction thereof and shows a flow-down condition where the ridges 3 of the absorbing liquid 2 are allowed to flow down as waving in the axial direction of the tube 1.
Thus, a smooth tube is conventionally used as the heat transfer tube for the absorber, and the Marangoni convection is generated by adding the surface active agent to the absorbing liquid to thereby improve the performance to some extent.
However, the time of residence of the absorbing liquid on the outer surface of the smooth tube depends both on the viscosity of the absorbing liquid and on the surface condition of the smooth tube. There is a limit for elongation of the residence time of the absorbing liquid, and a wetted area of the outer surface of the smooth tube to be wetted with the absorbing liquid cannot be widened over a certain degree. In addition, there is a limit of the absorbing power of the absorbing liquid. While the absorbing power of the absorbing liquid can be enlarged by increasing the turbulence of the absorbing liquid, the two actions, i.e., the residence and the turbulence of the absorbing liquids, are contrary to each other, so that the improvement of the heat transfer performance is limited.
To make the performance higher and the structure more compact in a heat exchanger, various developments of a high-performance unsmoothed heat transfer tube improved in its heat transfer performance have been made instead of the smooth tube. For example, a low-fin tube having a large heat transfer area has been proposed to obtain a high heat transfer performance. However, the use of the low-fin tube hinders the Marangoni convection on the outer surface of the tube, thus making the turbulence of the absorbing liquid insufficient to result in a decrease in absorption performance. Then, developments of such a tube form as not to hinder the Marangoni convection have been made.
For example, there has been proposed a fluted tube having axially extending grooves on the outer surface as a tube form not hindering the waving of the absorbing liquid in the axial direction of the tube (Japanese Patent Laid-open No. 63-6364). The conventional fluted tube having the grooves on the outer surface has a cross section such that the top of each crest portion and the bottom of each trough portion formed on the outer surface of the tube are acute. Accordingly, when the absorbing liquid moves over each crest portion, there occurs rapid shrinkage of the absorbing liquid, resulting in generation of a locally dried portion where the absorbing liquid is locally absent on the outer surface of the tube or resulting in generation of an extremely thin film portion of the absorbing liquid having such a film thickness as not to contribute to the absorption.
Such a locally dried portion and an extremely thin film portion hardly contribute to heat transfer, so that an effective improvement in the performance cannot be obtained. Furthermore, an excessively thick film portion is also formed because of the acute shape of each trough portion, thus reducing the turbulence effect by the convection, so that an effective improvement in the performance cannot be obtained.
In absorbing water vapor by arranging a given number of stages of heat transfer tubes, the absorbing liquid must be stayed on the outer surface of each heat transfer tube for a certain period of time to increase the amount of water vapor to be absorbed. However, if the absorbing liquid is stayed for an undue period of time, the concentration of the absorbing liquid at a position of residence is decreased to cause a reduction in absorption power, thus resulting in a reduction in heat transfer performance.